1. Field of the Invention
The present invention relates to a color picture tube and, more particularly, to a shadow mask supporting assembly.
2. Description of the Prior Art
In a conventional color picture tube, a shadow mask is mounted through spring members on stud pins embedded in the inner side wall surface of a panel of a rectangular shape. A phosphor screen is formed on the inner surface of the front side of the panel. Electron beams pass through predetermined apertures of the shadow mask and land at predetermined positions of the phosphor screen.
A typical conventional shadow mask supporting assembly in a color picture tube is illustrated in FIG. 1. The supporting assembly is a combination of stud pin 10 vertically extending on the inner side wall surface of panel 1 and spring member 13 having a through hole receiving pin 10. Pin 10 is tapered, and its tapering angle .theta. is normally about 12 degrees. In order to sufficiently compensate for mislanding of the electron beam on the phosphor screen caused by thermal expansion of shadow mask 7, movable part 13a forms an angle of about 55 degrees with respect to tube axis 14. By thermal expansion of the shadow mask, member 13 and mask frame 8 are moved to positions indicated by broken lines in FIG. 1. Mask 7 is slightly moved in the direction of screen 4 so that electron beam mislanding can be compensated.
In the supporting assembly for the shadow mask described above, when the color picture tube receives an impact or vibrations, member 13 is slid along pin 10 and is often fixed at a position different from the predetermined fitting position. In this case, the electron beam bombards a phosphor position different from the corresponding one degrading color purity. In the worst case, member 13 may be removed from pin 10.
In order to solve this problem, an elastic force of the spring member can be increased. However, in this case, the following problem is presented.
If a thermal expansion coefficient of a material constituting the stud pin is not substantially the same as that of the panel, cracks occur at that portion of the panel on which the stud pin is fixed, thus limiting the type of material which can be used for the stud pin. For example, a currently used material is exemplified by an Fe-Cr alloy (Cr: 18 wt%). This material is soft; it has a Vickers hardness (Hv) of 150. A typical spring member material is hard stainless steel having an Hv of 380 to 500. For this reason, if the spring force of the spring member is excessively large, or if an external impact is excessively large, the stud pin tends to be recessed. If the stud pin is recessed, the contact position of the spring member and the stud pin is shifted so that the position of the shadow mask is deviated to disable accurate landing of the electron beam on the phosphor screen.
Furthermore, since a large spring force is transmitted to the panel through the stud pin, the panel tends to be deformed. For this reason, the color picture tube is often damaged during normal annealing performed in the fabrication process of the color picture tube.
Furthermore, in the fabrication process of the color picture tube, the shadow mask must be repeatedly attached to or detached from the mask. If a large spring force acts on the panel, it is difficult to manually remove the shadow mask from the panel. Such detachment must be performed by a bulky detaching apparatus installed in the corresponding steps. In this case, a large pressure acts on the spring member, the mask frame, and the shadow mask (1) and thus may deform the mask frame or the shadow mask.